A fundamental biological process is to take in nutrients in the form of food and drink and to convert their chemical energy into macchanical energy and heat. Food is progressively broken down in the intestines until its constituents can pass through the gut wall and be absorbed into the blood. Most of the nutrients then pass to the liver, which controls their storage as glucose and glycogen, and as fat, the body's final energy reserve. when needed, first glucose and then glycogen again pass into the bloodstream as readily usable compounds, mainly sugars. Only a small proportion of the food is used for building up body tissues or reaches the adipose tissues as fat.